In general, commercial fluid catalytic cracking (FCC) processes are carried out in FCC units which are either side-by-side, where the unit includes a separate reactor vessel and a separate regenerator vessel adjacent to one another, or a stacked type of unit, where either the reactor vessel is stacked on top of the regenerator or vice versa. In these types of FCC units, the reactor is compartmentalized apart from the regenerator, meaning that there are separate overhead streams from both the reactor and the regenerator which require separate equipment to further treat each product stream.
Important to the FCC process is the location and control of the cracking reaction. Typically, FCC units have a discrete riser with some dense phase fluidized catalyst bed in the reactor vessel. With a highly active catalytic cracking catalyst, such as a zeolite catalyst, the dense phase bed height can be kept to a minimum, and primary control of the reaction can be accomplished by controlling the circulation rate of the catalyst.
Fresh feed and recycle streams in an FCC unit are typically preheated by heat exchangers or a furnace and enter the FCC unit in a feed riser which recirculates hot regenerated catalyst. The heat from the hot catalyst causes the fresh feed to vaporize. Vaporization may not be complete, however, if high boiling feeds are used or when large liquid droplets are present in the feed. Once the hydrocarbon vaporizes and mixes with the catalytic cracking catalyst, a fluid like suspension is formed, and the suspension is transported through the riser and to the reactor. As the feed contacts the hot catalyst in the riser, the cracking reaction begins. Typically, the riser empties the fluid like suspension of hydrocarbon and catalyst into the discrete phase bed in the reactor vessel, where a significant portion of the hydrocarbon is cracked.
Hydrocarbon cracking is a term which is well known in the art of petroleum refining and generally refers to the cracking of a large hydrocarbon molecule to a smaller hydrocarbon molecule by breaking at least one carbon to carbon bond. For example, large paraffin molecules can be cracked to a paraffin and an olefin, and a large olefin molecule can be cracked to two or more smaller olefin molecules. Long side chain molecules which may be present on aromatic rings or naphthenic rings can also be cracked.
As known in the art, catalytic cracking catalyst can be used to catalytically control a hydrocarbon cracking reaction. However, thermal cracking reactions also occur within the system. In addition, numerous side reactions accompany the cracking reactions. A few of the side reactions include dehydrogenation, cyclization, oligomerization, polymerization, hydrogen transfer and coke formation on the catalyst.
Hydrocarbon which is typically used as a feedstock for FCC units, contains sulfur and nitrogen in the form of organo-sulfur compounds and organo-nitrogen compounds. These types of compounds are quite extensive and are well documented. Examples of these compounds include compounds such as mercaptans, sulfides, disulfides, thiophenes, pyrroles and pyridines.
When a feedstock is cracked in an FCC unit, certain side reactions affect the sulfur and nitrogen components of the organo-sulfur and organo-nitrogen compounds in the hydrocarbon feedstream. Approximately 40-60% of the sulfur is converted to H.sub.2 S, about 35-55% of the sulfur remains in the liquid products, and about 5-10% of the sulfur ends up in the coke. The nitrogen in the feedstock is typically converted to NH.sub.3, amines, cyanides, and hetero cyclic type compounds. The feed nitrogen compounds can be either basic in nature or non-basic, with the total nitrogen percentage being generally in the range of 0.05-1.0 wt %. Typically, 20-80% of the nitrogen compounds are basic.
The cracking reaction continues in the reaction system until the hydrocarbon components are separated from the catalyst. Cyclones are generally used as the separation means. The remainder of the vapor is essentially entrained with the catalyst. At this stage, the catalyst is also adsorbed with coke which is to be removed so that the catalyst can be reused.
Substantially all of the NH.sub.3 and H.sub.2 S produced during the cracking reaction is recovered with the separated vapor in the reactor overhead. A portion of the nitrogen and sulfur compounds, however, are retained in the coke which is adsorbed to the catalyst.
The coked catalytic cracking catalyst is sent to the regenerator portion of the FCC unit for coke removal. In the regenerator, the coke is burned from the catalyst with an oxygen containing stream such as air. This burning reaction oxidizes the nitrogen and sulfur in the adsorbed coke forming, inter alia, NO, NO.sub.2, NO.sub.3, SO.sub.2 and SO.sub.3. If air is used as the oxygen containing stream, a significant portion of the nitrogen in the stream will also oxidize, forming NO, NO.sub.2 and NO.sub.3. Collectively, these compounds are referred to as NO.sub.x and SO.sub.x components.
After the NO.sub.x and SO.sub.x components are formed in the FCC regenerator, they are discharged as part of the flue gas stream. Because the NO.sub.x and SO.sub.x content of the flue gas stream may be at a higher concentration than is acceptable, the flue gas stream is typically further treated using additional equipment.
Due to the high temperatures and stresses placed on the catalytic cracking catalyst, attrition of the catalyst may occur. This attrition may produce catalyst fines (i.e., catalyst dust) which may also exit the catalytic cracking system in the regenerator off gas stream. Catalyst fines are catalyst particles which are less than about 40 .mu.m in diameter.
A problem inherent in known FCC units is that reduction of SO.sub.x and NO.sub.x components in the flue gas stream is costly in that much additional treatment is required. In addition, an excessive amount of catalyst fines in the flue gas stream can cause an undesirable increase in particulate emissions.
Another problem in known FCC units is that only a low concentration of oxygen in the oxygen stream can be tolerated in the regenerator for burning the coke from the catalyst. In general, if the oxygen content of the oxygen stream is too high, the temperature within the coked catalyst particles will be elevated to a point where there will be catalyst degradation.
Known FCC units typically use dense bed reaction and regeneration systems. These systems are not particularly desirable, however, since a relatively large amount of catalyst must be maintained in the FCC unit. If the amount of catalyst in the unit can be reduced, the entire system can be more easily controlled. This will result in more complete removal of coke from spent catalyst which leads to increased hydrocarbon conversion, meaning that the catalyst will be in a more active state. This also means that less fresh catalyst make-up will be required.
U.S. Pat. No. 4,464,250 is one example of a known FCC process. The process disclosed uses a part of the regeneration gas to strip hydrocarbon from the spent catalyst prior to regeneration. The stripping step is also performed in the dense phase.